The present invention relates to a light scanning device used for a laser beam printer, etc. More particularly, the present invention relates to a laser scanning device, and especially to a circuit configured to detect an SOS (start-of-scan) signal used as a synchronizing signal for determining laser scanning timing.
So as to obtain an SOS signal used as a timing signal while scanning a laser beam on a image plane, a laser scanning device is configured to generate the SOS signal based on a light receiving signal outputted from a photo diode (hereinafter, referred to as a PD) that receives the scanned laser beam. For instance, in a laser scanning device as indicated in FIG. 1, a laser beam LB emitted from a laser diode (hereinafter, referred to as an LD) 1 is deflected and scanned by a rapidly rotating polygon mirror 2 within a predetermined angle range, and main scanning of the LB directed to a photoconductive drum 4 through an fθ lens 3 is performed on a photosensitive surface of the axially-rotating photoconductive drum 4. Further, there is provided a PD 5 on a scanning optical path of the LB. The PD 5 receives the scanned LB and outputs the photoelectrically-converted light receiving signal. The SOS signal is obtained by processing the light receiving signal with an SOS signal generating circuit 6. On the basis of the SOS signal, a controlling circuit 7 controls an LD driving circuit 8, a polygon mirror driving circuit 9, and a photoconductive drum driving circuit 10 to make the laser scanning device start drawing with the laser beam a fixed time after receiving the SOS signal. This operation allows drawing on a specific area 4a of the photoconductive drum 4 with synchronized scanning timing.
As such an SOS signal generating circuit, conventionally, a technology has been proposed to compare the light receiving signal outputted from the PD receiving the LB with a reference potential. For example, an SOS signal generating circuit shown in FIG. 9 is configured to convert a light receiving current Io, which is outputted from a PD 5 when the PD 5 receives a laser beam to be scanned, to a light receiving voltage Vo by means of an I-V converting circuit 11 provided with an operational amplifier OP and a resistor R, and to output the SOS signal in accordance with comparison by a comparator 12 between the converted light receiving voltage Vo and a reference voltage Vref. In other words, the SOS signal generating circuit is configured to output the SOS signal from the comparator 12 when the light receiving voltage Vo is lower than the reference voltage Vref.
In such a technology, there is a problem that when fluctuations occur in the level of the light receiving current Io outputted from the PD 5 and/or the DC level (dark current, offset current) of the I-V converting circuit 11 that converts the light receiving current to the light receiving voltage Vo, the waveform of the SOS signal is deformed to cause timing deviation (phase shift in time domain) of the SOS signal. For instance, as shown in FIG. 10B, when the level of the light receiving current Io (that is, the negative level of the light receiving current Io with respect to the DC level thereof) outputted from the PD 5 decreases due to degradation of the LD 1 and/or the light receiving surface of the PD 5, the pulse width of the SOS signal is narrower than usual operation. Since the synchronous timing of the laser scanning device is defined by the rising and falling edges of the SOS signal, such fluctuation of the pulse width of the SOS signal directly leads to the deviation of the synchronous timing. In addition, as shown in FIG. 10C, when the DC level of the output from the I-V converting circuit 11 shifts in negative direction, the absolute value of the light receiving current Io (light receiving voltage Vo) is larger during the SOS signal, and thereby the pulse width of the SOS signal is larger. Similarly in this case, the fluctuation of the pulse width of the SOS signal results in the deviation of the synchronous timing.
As an example to solve one of the above deviations of the synchronous timing, which is especially caused by the fluctuation of the PD output, Japanese Patent Provisional Publication No. HEI11-160636 discloses a technique to adjust the reference voltage of the comparator by a CPU which controls the LD to change the image deepness. The reference voltage of the comparator is adjusted by the CPU in order to compensate the deviation of the synchronous timing that is caused by the deformation of the SOS signal waveform according to the level fluctuation of the light receiving signal outputted from a synchronizing sensor, i.e., the PD when the image deepness is changed. Thereby, the reference voltage is shifted according to the fluctuation of the PD output to compensate the deviation of the synchronous timing due to the deformation of the SOS signal waveform.
Since the above technique is adapted to shift the reference voltage of the comparator according to the fluctuation of the light receiving signal outputted from the PD when the image deepness is changed, it can compensate the deviation of the synchronous timing in the case where the rising or falling of the level of the light receiving signal from the PD is previously known. However, it cannot support the deviation of the synchronous timing that is caused by uncertain factors such as the level fluctuation of the PD output depending on temperature and/or time and the DC level shift of the I-V converting circuit.